Dielectric ceramic composition

ABSTRACT

There is provided a dielectric ceramic composition which is composed mainly of PbTiO 3 , Pb(Mg 1/3  Nb 2/3 )O 3 , and Pb(Fe 2/3  W 1/3 )O 3 , with 0.5 to 10 mol % of Pb atoms partly replaced by Ba atoms and/or Sr atoms as represented by the formula (I) below. 
     
         (Pb.sub.1-u M.sub.u)[Ti.sub.x (Mg.sub.1/3 Nb.sub.2/3).sub.y (Fe.sub.2/3 
    
      W 1/3 ) z  ]O 3                                  (I) 
     where 
     M denotes Ba and/or Sr, 
     0.005≦u≦0.10, 
     0.10≦x≦0.40, 
     0.05≦y≦0.85, 
     0.05≦z≦0.55.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a dielectric ceramic composition. More particularly, it is concerned with a dielectric ceramic composition suitable as a dielectric material for high-capacity capacitors or the like on account of its high dielectric constant, outstanding temperature characteristics of dielectric constant, and low firing temperature.

Heretofore, the high-dielectric ceramic composition has been dominated by the one composed mainly of barium titanate (BaTiO₃). The BaTiO₃ -based composition generally contains a shifter to shift the Curie point to the vicinity of room temperature, a depressor to improve the capacitance-temperature characteristics, a sintering promoter, and an anti-reducing agent.

Examples of the shifter include BaSnO₃, BaZrO₃, CaZrO₃, CaSnO₃, SrTiO₃, PbTiO₃, La₂ O₃, and CeO₂. Examples of the depressor include CaTiO₃, MgTiO₃, Bi₂ (SnO₃)₃, Bi₂ (TiO₃)₃, NiSnO₃, MgZrO₃, and MgSnO₃. Examples of the sintering promoter include Al₂ O₃, SiO₂, ZnO, CeO₂, B₂ O₃, Nb₂ O₅, and WO₃. Examples of the anti-reducing agent include MnO₂, Fe₂ O₃, and CuO.

Unfortunately, the BaTiO₃ -based composition has to be fired at a high temperature (1300° to 1400° C.). This leads to a high firing cost. In addition, when the composition is applied to multilayer ceramic capacitors, the high firing temperature makes it necessary to use an expensive noble metal such as palladium and platinum as internal electrodes, which prevents the cost reduction of multilayer ceramic capacitors.

To overcome this disadvantages, there have been developed those materials shown in Table 1 which are capable of firing at 1100° C. or below and permit the use of comparatively less expensive metals such as silver as internal electrodes.

                                      Table 1                                      __________________________________________________________________________                             Firing tem-                                                                          Dielectric                                                                           Dielectric                                                                           Resistivity                          Laid-open               perature                                                                             constant                                                                             loss  ρ                                No.  Composition        (°C.)                                                                         ε.sub.5                                                                      tan δ (%)                                                                      (Ω · cm)              __________________________________________________________________________     56-103802                                                                           Pb[Ti(Mg.sub.1/3 Nb.sub.2/3 )(Fe.sub.2/3 W.sub.1/3)]O.sub.3                                       950   16000 3.0   --                                        MnO.sub.2 or Co.sub.2 O.sub.3                                             55-121959                                                                           (Pb,M)[Ti(Mg.sub.1/3 Nb.sub.2/3)]O.sub.3 M = Ba, Sr,                                              --    24700 0.7   --                                   57-25606                                                                            Pb[Ti(Mg.sub.1/3 Nb.sub.170 )]O.sub.3                                                             1100  23000 1.25  --                                   58-89707                                                                            Pb[Ti(Fe.sub.170 W.sub.1/3)(Mn.sub.1/3 Nb.sub.170 )]O.sub.3                                       --    11000 0.57  8 × 10.sup.11                  55-144612                                                                           Pb[Ti(Mg.sub.1/3 Nb.sub.2/3 )(Mg.sub.1/2 W.sub.1/2)                                               2/3 1000                                                                              9830 0.3   2 × 10.sup.11                       (Mn.sub.1/3 Nb.sub.2/3)]O.sub.3                                           57-67211                                                                            Pb[Ti(Zn.sub.1/3 Nb.sub.2/3)(Fe.sub.2/3 W.sub.1/3)]O.sub.3                                        920   16200 0.66  5 × 10.sup.11                  59-105207                                                                           Pb[(Ni.sub. 1/3 Nb.sub.2/3)(Zn.sub.1/3 Nb.sub.2/3)(Fe.sub.2/3                  W.sub.1/3)]O.sub.3 880    4100 0.44  4.6 × 10.sup.12                __________________________________________________________________________

The high-performance dielectric ceramic compositions proposed so far are not necessarily satisfactory in all of dielectric constant, dielectric loss, and resistivity as shown in Table 1. In addition, they are poor in the temperature characteristics of dielectric constant. For this reason, there has been a demand for a dielectric ceramic composition which is capable of firing at a low temperature and yet satisfactory in all of dielectric constant, dielectric loss, resistivity, and temperature characteristics of dielectric constant.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a dielectric ceramic composition which has a high dielectric constant, a low dielectric loss, and a high resistivity.

It is another object of the present invention to provide a dielectric ceramic composition which can be made at a low cost into small-sized large-capacity capacitors of high reliability.

It is further another object of the present invention to provide a dielectric ceramic composition which is capable of firing at a low temperature and hence permits the use of inexpensive silver as the internal electrodes of multilayer ceramic capacitors, contributing to the reduction of the production cost of multilayer ceramic capacitors.

The dielectric ceramic composition of the present invention is composed mainly of PbTiO₃, Pb(Mg_(1/3) Nb_(2/3))O₃, and Pb(Fe_(2/3) W_(1/3))O₃, with Pb atoms partly replaced by Ba atoms and/or Sr atoms. Thus the dielectric composition is represented by the formula (I) below.

    (Pb.sub.1-u M.sub.u)[Ti.sub.x (Mg.sub.1/3 Nb.sub.2/3).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z ]O.sub.3                                 (I)

where

M denotes Ba and/or Sr,

0.005≦u≦0.10,

0.10≦x≦0.40,

0.05≦y≦0.85,

0.05≦z≦0.55,

preferably

0.02≦u≦0.08,

0.15≦x≦0.35,

0.20≦y≦0.75,

0.10≦z≦0.36.

In addition, the dielectric composition of the present invention may contain Pb(Mn_(1/3) Nb_(2/3))O₃ in an amount of 5 mol %, preferably 0.02 to 3 mol %, of the major components.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The composition of the present invention is represented by the formula below

ti Pb[Ti_(x) (Mg_(1/3) Nb_(2/3))_(y) (Fe_(2/3) W_(1/3))_(z) ]O₃

(where x, y, and z are defined as above) in which Pb atoms are partly replaced by Ba atoms or Sr atoms as shown in the formula (I) above. Therefore, the composition has an extremely low dielectric loss and a high resistivity.

With the value of u lower than 0.005 in the formula (I), the composition does not exhibit the desired properties. With the value of u higher than 0.10, the composition has a low dielectric constant. Consequently, the value of u should be in the range of from 0.005 to 0.10, preferably from 0.02 to 0.08.

With the value of x lower than 0.10 in the formula (I), the composition has a low dielectric constant. With the value of x higher than 0.40, the composition has a low resistivity and a high dielectric loss. Consequently, the value of x should be in the range of from 0.10 to 0.40, preferably from 0.15 to 0.35.

With the value of y lower than 0.05 in the formula (I), the composition has a low resistivity. With the value of y higher than 0.85, the composition has a low dielectric constant. Consequently, the value of y should be in the range of from 0.05 to 0.85, preferably from 0.20 to 0.75.

With the value of z lower than 0.05 in the formula (I), the composition has a high firing temperature and a low dielectric constant. With the value of z higher than 0.55, the composition has a low resistivity. Consequently, the value of z should be in the range of from 0.05 to 0.55, preferably from 0.10 to 0.36.

The dielectric composition of the present invention may be incorporated with Pb(Mn_(1/3) Nb_(2/3))O₃ to increase the dielectric constant and resistivity and decrease the dielectric loss. To produce this effect, the amount to be added should be less than 5 mol %, preferably 0.02 to 3 mol %, of the major components.

The dielectric composition of the present invention is produced in the following manner. At first, raw material powders such as lead monoxide, titanium dioxide, barium titanate, strontium carbonate, magnesium oxide, niobium pentoxide, ferric oxide, tungsten trioxide, and manganese dioxide are weighed according to a prescribed ratio. They are thoroughly mixed by wet process using a ball mill or the like. The resulting mixture is dried and, if necessary, calcined at 600° to 800° C. for several hours. This calcination is not a must; but it makes the particle size uniform and improves the dielectric properties. The calcined raw material powder is pulverized further by wet process using a ball mill or the like. After drying, the powder is granulated with an organic binder such as polyvinyl alcohol. The granules are press-formed into a desired shape. The shaped item is fired at 950° to 1050° C. for half an hour to several hours. (The above-mentioned numerical values are merely exemplary; and the composition may be produced under other conditions.)

The dielectric composition of the present invention has a high dielectric constant, a low dielectric loss, and a high resistivity. Therefore, it can be made into small-sized, high-capacity capacitors of high reliability.

In addition, the dielectric composition of the present invention can be fired at low temperatures (950° to 1050° C.). This leads to a low firing cost and permits the use of comparatively less expensive metals (e.g., silver) in place of expensive noble metals (e.g., palladium and platinum) for the internal electrodes when it is made into laminated capacitors. This, in turn, contributes to a great reduction in the production cost of multilayer ceramic capacitors.

The invention is now described in more detail with reference to the following examples, which are not intended to restrict the scope of the invention.

EXAMPLE 1

Dielectric ceramic compositions were prepared from PbO, BaTiO₃, SrCO₃, TiO₂, MgO, Nb₂ O₅, Fe₂ O₃, WO₃, and MnO₂. They were weighed according to the mixing ratio shown in Table 2 and then mixed by wet process for 20 hours using a ball mill. The resulting mixture was dehydrated, dried, and calcined at 750° C. for 2 hours. The calcined product was pulverized again by wet process for 20 hours using a ball mill, followed by dehydration and drying.

The thus obtained powder was mixed with an aqueous solution of polyvinyl alcohol, and the wet powder was compression-molded into disks, 16 mm in diameter and 0.8 mm thick, under a pressure of 3 tons/cm². The molded disks were fired at 950° to 1050° C. for 2 hours in a magnesia ceramic vessel.

Each of the fired disks was provided on both sides thereof with silver electrode by firing at 650° to 700° C. The disks were examined for electrical properties. The results are shown in Table 2.

The dielectric constant and dielectric loss were measured with a YHP digital LCR meter, Model 4274A, at a frequency of 1 kHz and a voltage of 1.0 V_(rms) and at a temperature of 25° C. The resistivity was measured with a YHP meter, Model 4329A, at 25° C. Readings were taken one minute after the application of 100 V. In Table 2, ε₋₂₅, ε₂₅, and ε₈₅ denote the dielectric constant at -25° C., 25° C., and 85° C., respectively, and ε_(max) denotes the maximum value of dielectric constant in the temperature range of -25° C. to 85° C.

It is noted from Table 2 that the dielectric ceramic compositions of the present invention have a high dielectric constant, a low dielectric loss, a high resistivity, and outstanding temperature characteristics of dielectric constant.

    TABLE (2)1       Sample      Pb(Mn.sub.1/3       Nb.sub.2/3)O.sub.3 Firing Dielectric Dielectric Resisitivity (E.sub.25      -E.sub.25)/E.sub.25 (E.sub.max -E.sub.25)/E.sub.25 (E.sub.85       -E.sub.25)/E.sub.25  No. x y z M u (mol %) temp. (°C.) constant      loss (%) (Ω.cm) × 100 × 100 ×       100 Note             1 0.05 0.75 0.20 Ba 0.02  1000 7200 0.76 4.6      × 10.sup.12 49.0 49.0 -37.4 C.E. 2 0.05 0.85 0.10 Ba 0.05  1050      7700 0.10 7.3 × 10.sup.12 42.4 42.4 -43.9 C.E. 3 0.09 0.61 0.30 Ba      0.05  1000 8500 0.10 6.5 × 10.sup.12 52.2 52.2 -45.7 C.E. 4 0.12      0.78 0.10 Ba 0.05  1050 9800 0.25 8.5 × 10.sup.12 -5.7 6.7 -28.8      W.E. 5 0.15 0.55 0.30 Sr 0.003 1.0 1000 14400  2.14 8.3 ×       10.sup.10 -61.0 1.7 -57.6 C.E. 6 0.15 0.65 0.20 Ba 0.02  1000 12100      0.91 3.4 × 10.sup.12 -41.4 2.1 -37.9 W.E. 7 0.15 0.65 0.20 Ba 0.02      0.5 1000 13000  0.74 6.1 × 10.sup.12 -40.1 1.5 -38.3 W.E. 8 0.15      0.75 0.10 Ba 0.08  1050 9600 0.97 8.0 × 10.sup.11 -36.1 2.3 -30.8      W.E. 9 0.18 0.52 0.30 Ba 0.05  1000 12400  0.27 8.5 × 10.sup.11      6.8 18.8 -45.6 W.E. 10 0.20 0.45 0.35 Ba 0.002 1.0 1000 12900  2.51 2.0      × 10.sup.11 -52.9 3.1 -41.7 C.E. 11 0.20 0.70 0.10 Ba 0.05  1050      9000 0.47 5.3 × 10.sup.12 -40.1 12.8 -19.7 W.E. 12 0.20 0.78 0.02      Ba 0.05  1100 7500 0.61 6.5 × 10.sup.12 -46.6 15.0 -2.3 C.E 13      0.23 0.42 0.35 Ba 0.02  1000 14200 0.92 3.4 × 10.sup.11 -44.2 2.3      -32.6 W.E. 14 0.23 0.42 0.35 Ba 0.02 1.0 1000 15600 0.86 2.0 ×      10.sup.12 -46.1 1.7 -30.9 W.E. 15 0.24 0.46 0.30 Sr 0.03 1.0 1000 16000      0.45 1.3 × 10.sup.12 -36.5 1.2 -34.7 W.E. 16 0.24 0.46 0.30 Ba      0.05 0.5 1000 14900 0.26 2.1 × 10.sup.12 -19.6 5.1 -33.8 W.E. 17      0.24 0.46 0.30 Ba 0.057.0 1000 12900 2.52 7.8 × 10.sup.11 -56.2      7.2 -26.9 C.E. 18 0.25 0.35 0.40 Ba 0.02 2.0  950 13200 0.79 1.2 ×      10.sup.12 -54.5 5.9 -24.4 W.E. 19 0.25 0.50 0.25    1000  5300 3.61 2.1      × 10.sup.10 -29.7 50.1 50.1 C.E. 20 0.25 0.55 0.20 Ba 0.15  1000      7200 0.36 2.1 × 10.sup.12 -23.6 2.7 -19.1 C.E. 21 0.25 0.75  Ba      0.05  1150  7100 2.10 7.2 × 10.sup.12 -42.1 32.5 29.8 C.E. 22 0.26      0.39 0.35 Ba 0.02 1.0 1000 13200 0.88 3.8 × 10.sup.12 -47.0 18.6      -10.4 W.E. 23 0.26 0.39 0.35 Ba 0.08 1.0 1000 11800 0.29 6.0 ×      10.sup.11 18.8 19.8 -44.4 W.E. 24 0.30 0.30 0.40 Ba 0.05 1.0  950 13600      0.33 1.5 × 10.sup.12 -53.7 0.2 - 39.9 W.E. 25 0.30 0.35 0.35 Ba      0.05 1.0 1000 14000 0.35 1.0 × 10.sup.12 -54.5 6.1 -22.3 W.E. 26      0.30 0.10 0.60 Ba 0.05   950  8200 0.92 1.2 × 10.sup.10 8.1 26.3      -52.0 C.E. 27 0.30 0.50 0.20 Ba 0.05 1.0 1000 10400 0.62 3.6 ×      10.sup.12 -47.9 12.0 -5.3 W.E. 28 0.32 0.23 0.45 Ba 0.05 3.0  950 12500      0.56 7.8 × 10.sup.11 -44.9 4.1 --20.6 W.E. 29 0.32 0.28 0.40 Sr      0.05   950 10600 0.61 4.3 × 10.sup.11 -47.6 5.5 -42.9 W.E. 30 0.32      0.28 0.40 Sr 0.05 1.0  950 12100 0.47 1.1 × 10.sup.12 -45.3 4.7      -43.0 W.E. 31 0.32 0.33 0.35 Ba 0.05 3.0 1000 13300 0.52 8.0 ×      10.sup.11 -43.8 6.5 -15.0 W.E. 32 0.35 0.53 0.12 Sr 0.08 1.0 1050  9200      0.73 2.1 × 10.sup.12 -52.8 0.6 -26.0 W.E. 33 0.45 0.10 0.45 Ba      0.05 3.0  950  8800 1.56 6.0 × 10.sup.10 -38.2 41.7 41.7 C.E. 34      0.45 0.03 0.52 Ba 0.05 3.0  950  8100 1.34 4.3 × 10.sup.10 -45.1      33.6 28.4 C.E. 35 0.45 0.40 0.15 Sr 0.12  1050  6400 1.52 4.3 ×      10.sup.11 -32.2 7.0 -15.7 C.E.      Remarks:?      Columns 2 to 6 show the numerical values in the formula (l).      Columns 12 to 14 show the temperature characteristics of dielectric      constant (%).      W.E. = Working Examples, C.E. = Comparitive Examples 

What is claimed is:
 1. A dielectric ceramic composition comprising PbTiO₃, Pb(Mg_(1/3) Nb_(2/3))O₃, and Pb(Fe_(2/3) W_(1/3))O₃, with 0.5 to 10 mol % of Pb atoms partly replaced by at least one of Ba atoms and Sr atoms as represented by the formula (I) below:

    (Pb.sub.l-u M.sub.u)[Ti.sub.x (Mg.sub.1/3 Nb.sub.2/3).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z ]O.sub.3                                 (I)

where M denotes at least one of Ba and Sr, 0.005≦u≦0.10, 0.10≦x≦0.40, 0.05≦y≦0.85, 0.05≦z≦0.55, and Pb(Mn_(1/3) Nb_(2/3))O₃ in an amount more than 0 an mol % of said above components.
 2. A dielectric ceramic composition as claimed in claim 1, wherein the value of u in the formula (I) is in the range of 0.02≦u≦0.08.
 3. A dielectric ceramic composition as claimed in claim 1, wherein the value of x in the formula (I) is in the range of 0.15≦x≦0.35.
 4. A dielectric ceramic composition as claimed in claim 1, wherein the value of y in the formula (I) is in the range of 0.20≦y≦0.75.
 5. A dielectric ceramic composition as claimed in claim 1, wherein the value of z in the formula (I) is in the range of 0.10≦z≦0.36.
 6. A dielectric ceramic composition as claimed in claim 1 wherein Pb(Mn_(1/3) Nb_(2/3))O₃ is contained in an amount of 0.02 to 3 mol %. 